Robin Roberts, co-host of ABC's "Good Morning America," revealed that she has been diagnosed with MDS (myelodysplastic syndrome) five years after successfully beating breast cancer.

On Monday a teary-eyed Roberts shared her diagnosis during an emotional segment on "GMA," and while she remains confident that she will beat the disease, she has requested prayers.

"I've always been a fighter, and with all of your prayers and support, a winner...My doctors tell me I'm going to beat this -- and I know it's true," Roberts wrote on gma.yahoo.com.

"When I faced breast cancer, your prayers and good wishes sustained me, gave me such hope and played a major role in my recovery. In facing this new challenge, I ask humbly for more of your prayers and love – as I will keep you in my mine and update you regularly on my condition," she wrote.

Roberts, who attributes her strength to her Christian faith, has been battling a rare blood disorder that affects the bone marrow. While the disease usually arises on its own, in Roberts' case it occurred as a result of her 2008 cancer treatment.

The 51-year-old begun pre-treatment on Monday, which involves taking a drug over the next few months to prepare her bone marrow for transplant later this year.

"Bone marrow donors are scarce and particularly for African-American women. I am very fortunate to have a sister who is an excellent match, and this greatly improves my chances for a cure," Roberts wrote.

In May, Roberts secured what would go on to become a controversial interview with President Barack Obama which explored his evolved views on same-sex marriage. Despite the good news, just one day prior she was faced with an "unpleasant" procedure to extract bone marrow from her body for testing.

"The combination of landing the biggest interview of my career and having a drill in my back reminds me that God only gives us what we can handle and that it helps to have a good sense of humor when we run smack into the absurdity of life," Roberts wrote. In 2008, Roberts completed eight chemotherapy treatments followed by 6-and-a-half weeks of radiation treatment before finally beating breast cancer.

Roberts, along with her doctors, remain confident that she will overcome the disease and go on to live a full and healthy life.

"Bottom line: I've been living with this diagnosis for awhile and will continue to anchor GMA. I love what I do and the people with whom I do it. Along with my faith, family and friends, all of you at ABC News give me the motivation and energy to face this challenge," Roberts wrote.